The present invention relates to a power supply normally powered by an alternating current line but switchable to direct current power and, in particular, to a circuit for preventing back feed into the alternating current power line.
Standby or uninterruptible power supplies for computer, cable TV, and other applications operate by switching to an internal, inverter power source (operating from batteries) when utility power is unavailable. Typically, such power supplies use an input transformer having two isolated primary windings, one of which may be driven by the alternating current power line, the other capable of being driven by an inverter circuit.
The presence of either of these two power sources on its respective primary winding produces a voltage on the other primary winding, due to normal transformer action. Typically, the inverter operates at low voltage, so the voltage induced on the inverter primary winding is in fact useful for purposes such as battery charging and powering control circuitry.
When the inverter is operating, however, a potentially hazardous voltage appears at the line primary winding, due to the turns ratio between the inverter and line primary windings. Normally, this inverter operation occurs only during a power failure, so the alternating current power line is open-circuited at some point. A repair technician or lineman may be subjected to this hazardous voltage, since they are working on what they believe to be a "dead" line.
In order to prevent this problem, it is required that the line primary winding be positively disconnected from the alternating current power line when the inverter is operating. This is normally done with a double-pole relay or contactor. One pole of the relay is normally closed and is used to switch the line, while the other pole is normally open and is used to switch the inverter battery supply. If no power is applied to the relay coil, the power supply operates from the power line, and the inverter is not connected to its battery. In the event of an alternating current power line failure, control circuits apply power to the relay coil, disconnecting the alternating current power line and connecting the inverter to its battery. It should be impossible for the inverter to operate unless the relay is pulled in and thus the power line isolated from the line primary winding.
There are two problems with this approach. First, with many contactor or relay designs, it is possible for a contact to weld in one position, and there is enough free play in the armature for the other contact to be pulled into the alternate position. In other words, it is possible for the line pole to weld stuck in the normally closed position. When power is applied to the relay coil, the armature can still move enough for the inverter pole to transfer to the closed position, applying power to the inverter. The welded or stuck contact has defeated the safety purpose of the relay.
In addition, the inverter pole must be rated to handle the full battery current required by the inverter. In a typical system, with a 36 volt nominal battery, the line pole may handle a maximum of 12 amps, while the inverter pole must handle on the order of 40-45 amps (due to constant power output from the system, but reduced input voltage). This results in a rather large and expensive relay or contactor.